Heater



Aug. 11, 1959 O. CAMPBELL ET AL HEATER 4 Sheets-Sheet 1 Filed July 22,1957 INVENTORS Oliver Campbell a Nqrman E. Pennels ATTORNEYS o. CAMPBELLETL 2,898,892

Aug. 11, 1959 HEATER 4 Sheets-Sheet 2 Filed July 22, 1957 INVENTORSOliver Campbell BY Normon E. Pennels ATTORNEYS Augo 1959 o. CAMPBELLETAL 2,898,892

HEATER 4 Sheets-Sheet 3 Filed July -22 1957 IN VENTORS O r m mm %WQ Ma%QA 1959 o. CAMPBELL ETAL 2,898,892

HEATER 4 Sheets-Sheet 4 Filed July 22, 1957 INVENTORS Oliver Campbell BYNorman E. Pennels l 2,898,892 HEATER Oliver Campbell, Whiting, Ind., andNorman E. Pennels, Lansing, Ill., assignors to Sinclair RefiningCompany, New York, N.Y., a corporation of Maine Application July 22,1957, Serial No. 673,391 '5 Clains. (Cl. 122-356) Our invention relatesto heating fluid materials and in particular provides a heater for avariety of materials commonly encountered in oil refinery service, suchas fixed petroleum gases, hydrogen, gas oil and residual ols.

It is a principal object of our invention to provide an oil refineryheater of the tubular type having a direct fired, radiant heatingsection in which the heat flux distribution about any given heatingtube, particularly at the high temperature end of each group of seriallyinterconnected heating tubes is substantially uniform.

It `is a further obejct of our invention to provide an oil refineryheater providing maximum utilization of the space within the furnaceshell for the location of double fired heating tubes.

It is still another important object of our invention to provide a'heating tube positioning arrangement in a refinery heater permittingdirect interconnecton of a group of freely supported, seriallyinterconnected radiantly heated tubes with a group of freely supported,serially interconnected convection heated tubes by which differences inexpansion of the hotter, radiantly heated tubes and the cooler,convection heated tubes are accommodated.

These and other objects of our invention are basically obtainedutilizing a vertical, cylindrical heater construction having an annular,upshot radiant heating furnace section with a central flue extendingdownwardly through such heating section and in which there are aplurality of upshot bumers spaced about the floor of the radiant heatingsection. In our heater we employ serally interconnected, verticallypositioned heating tubes which extend in single rows in the annularfurnace section radially from the flue between adjacent pairs of burnersas spokes extend from the hub of a wheel thus providing double ring ofeach of the heating tubes and assurng minimum ratios of heat release toheating capacity. Each radial row, i.e., bank, of heating tubes can atits inner end include a bank of one or more single fired verticalheating tubes located adjacent to the Wall of the flue and is connectedat its inner, inlet end in the upper part of the furnace above the flue-to the outlet end of a bank of one or more vertical heating tubes whichextend into the flue where it or they are heated by convection of theflue gases. The heating tube or tubes which are located in the flue arethus utilized for preheating to reduce the number of tubes in any singlepass through the heater which must be located in the radiantly heatedfurnace sections thereby requiring only a single radial bank of doublefired tubes in each pass and avoiding the necessity of interconnectingseveral of the radial banks in a single pass.

In a more specific aspect we contemplate supporting all heating tubes,both those in the convection section and those in the radiant section,only at the bottom of the tubes thus permittng expanson of the tubes asthey are heated to take place in an upward direction. By making theoutlet tubes in the convection section longer than their associatedtubes in the radiant furnace section, that is, by supporting the outletconnection tubes at a level below the floor of the annular radiantlyheated furnace section and by connecting the inlet tube of eachradiantly heated bank at its upper end to the upper end of theassociated outlet convection tube the fact that the convection tube willbe heated to a lower temperature and hence will expand proportionatelyless than their associated tates Patent O tubes in the furnace sectionsis partly obviated by the greater length of the outlet convection heatedtube which is connected to the direct fired bank and partly obviated bythe fact that the tubes are free at their upper ends and hence can movelaterally as well as vertically to accommodate any actual difference intheir total expansions.

For a more complete understanding of the practcal application of theprinciples of our invention reference is made to the appended drawingsin which: I

Figure l is a vertical section of an oil refinery heater constructed inaccordance with our invention taken at line 1-1 in Figure 2;

Figure 2 is a cross-section of the heater shown in Figure 1 taken atline 2-2 of Figure l;

Figure 3 is a partially sectioned, plan View of the heater shown inFigure l with the sectioned portion taken along line 3 3 in Figure 1;

Figure 4 is an enlarged, fragmentary sometric view of a portion of theheater shown in Figure 1;

Figure S is an enlarged fragmentary oblque View of another portion ofthe heater shown in Figure 1;

Figure 6 is a cross-section similar to Figure 2 illustrating amodification of the heater shown in Figure 1; and

Figure 7 is a fragmentary enlarged isometric view of 'anothermodification in Construction of a heater shown in Figure 1.

Referring to the drawings the reference numeral 10 generally desgnates acylindrical, upshot oil refinery heater which includes a cylindrical,vertical steel shell 11 completely lined with a layer 12 of refractorybrick. Shell 11 is positioned inside of four supporting steel columns 13attached to shell 11 at intervals and mounted on concrete footings 14.Positioned coaxially within shell 11 is a cylindrical upper flue section15 constructed of refractory material which terminates at its open upperend 16 in the upper portion of shell 11 substantially below the upperend of shell 11. Flue section 15 is supported on four steel columns 16mounted on concrete footings with its lower end at approxmately the samelevel as the lower end of shell 11. Each adjacent pair of columns 16 and13 have'afl'lxed between them horizontal girders 17 which support arefractory floor 18 of annular shape and which extends across the lowerend of shell 11 closing the lower end of shell 11 upon the lower end of'upper flue section 15.

Upper flue section 15 is attached at its lower end to the upper end of alower flue section 19, also of generally cylindrical shape andconstructed of refractory material. Lower flue section 19 extendsdownwardly between columns 16 between which it is supported by crossedhorizontal girders 20 affixed between diagonally opposite columns 16 attheir lower ends. The lower end of lower flue section 19 is enlarged toone `side to a sli ghtly larger diameter just above the bottom ofsection 19 as indicated by the reference numeral 21 (see particularlyFigures 1 and 3). On its side opposte enlarged portion 21, lower uesection 19 opens into a horizontal duct 22 of generally rectangularcross-section which leads laterallyinto an elbow box 23, thencerearwardly to a waste heat boiler 24 of conventional Construction and toexhaust blower 25 driven by a motor 26 which vents into a stack 27.

It Will be noted that the enlargement 21 of the lower portion of lowerflue section 19 and the horizontal position of duct 22 beneath heater 10necessitates passing certain of girders 13 and 16. Since the temperatureof the exhaust gases carried through duct 22 is still sufliciently highto recover useful heat, by waste heat boiler 24 for example, it isnecessary to protect such girders 13 and 16 from the heat where they arerequired to extend through lower flue section 20 and duet 22. We havefound particularly where I- or H-columns are employed, that such heatprotection is best atforded by building a 3 chimney 28 of refractorybrick enclosing the column 16 or 13 through the height of duct 22leaving open the space between the flanges of the column such that airfreely can convect'upwardly in such spaces assisting'in cooling thecolumn. i The general' structure of heater -104is completed by arefractory, roof 29 closing the upper end of shell 11 which suitably issupported-by and beneath steel girders 30 extendng between columns 13and other horizontal cross members indicatedjschematically in thedrawing. Centrally roof 29 is provided with a vertical, cylindricalsteel port 31 supported' asa hub at the inner ends of horizontal girders30 and which'is provided with a refractory lining 32. A manhole cover33; rests over port 31 and is similarly provided with an innerrefractory lining 34.

- v Referring more' particularly to Figures 1 and 2, it will be observedthat eight burners 35 are located in floor 18, generally equallydisposed 'between upper flue section 15 andlining 12 of shell 11 andspaced at equal arcuate intervals (45 degrees) apart concentrically withannular floor 18. Each-burner 35 includes an air register 36 on theunder side of floor 18 which can have any conventio'nal form and whichcan be connected to air preheating systems and the like, if desired.Above each register 36 a conical ho'le 37 is' cut in refractory floor 18thus defining a nozzle for burner 35. A fuel pipe 38 connected to asuitable fuel system and valves of conventional Construction is broughtthrough air register 38 upwardly to the center of conical opening 37where pipe 38 terminates. Thus gas or other fuels delivered through pipe38 when ignited are fired upwardly into the annular furnace chamber 40formed in heater 10 above floor 18 between lining 12 of shell 11 andrefractory upper flue section 15. Additional burners 41 can be mountedin shell 11 above each of burners 35 to provide auxiliary heat releasefor the upper furnace section 42 which is defined within lining 12 ofshell 11 between roof 29 and the upper end 16 of upper flue section 15.

Heating tubes are connected in eight separate passes .through heater 10leading from a common header 43 to a common outlet pipe 44.

A Header 43 desirably is located adjacent to the enlarged side 21 at thebase of lower flue section 19 and is con- .nected to the eight passesthrough heater 10 by separate valves 45 which lead to inlet pipes 46which extend through enla'rged portion 21. A number of heating tubes 47which stand vertically are packed into the flue gas well 48 defined influe sections' 15 and 19 and are supported at their lower ends inrefractory saddles 49 which -rest on a platform 50 supportedhorizontally in the base of lower flue section 19 upon legs '1 whichrest at their lower ends on girders 20. Platform 50 thus holds the lowerends of tubes 47 at about the middle level of duct 22. Because lowerflue section 19 is enlarged at its base -21 opposite duct22 and becauseplatform 50 is elevated, 'flue gas traveling downwardly through flue gaswell 48 is prevented from channeling at the lower ends of tubes 47toward duct 22. Thus the flue gases are distributed equally at the baseof tubes 47 passing in part directly to duct 22 and in part throughenlarged base portion 21 and thereafter under platform 50 into duct 22.

Heating tubes 47 are serially nterconnected in eight `separate banks A,each including an inlet tube 47' and an outlet tube 47". Each of inletpipes 46, as will' be seen more clearly in Figure 3, extends beneathplatform 50 .and is turned upwardly to connect with an inlet heatingtube 47'. Each of the heating tubes 47 in each serially nterconnectedbank A extends entirely in flue gas we'll 48,

.except that in each serially nterconnected bank A the out- -let -heatngtube 47" extends upwardly 4 heating tube 47 which is to be linked to anadjacent tube similarly provided with a U -bolt 53. Each pair ofadjacent U-bolts 53 carrying -between them a linking ring 54 whichpasses through U-bolts 53 with substantial clearance. The stack of tubes47 are thus held in their positional relationship to each other *but arefree to. move laterally within limits defined by the' clearances ofrings 54 in their associated U-bolts 53. V

An additional number of serially nterconnected vertical heating tubes 55are positioned in annular furnace section 40- resting .at the joinedlower ends 55a of adjacent pars in saddles- '56 formed in refractoryflooring 18. Tubes 55 also eXtend at their upper ends into upper furnacesection 42and are arranged in eight serially interconnected groups, eachgroup being disposed in a radial bank B consisting of a single''ow oftubes 55 extending from upper flue'section 16 outwardly toward shell 11and ashort arcuate bank C of asingle row of tubes 55 disposed adjacentto the outer wall of upper flue section 15.

The inlet tube 55' of each serially nterconnected group of tubes 55including a bank B and a bank C is located at the end of bank C remotefrom its associated bank B and extends upwardly to approximately thesame level as the outlet pipe 47" of a bank A of convection sectionheating tubes 47 towhich tube 55' is connected at its upper end'. Thusthe outlet tube 55" of each serially nterconnected group off-heatingtubes 55 is the outermost heating tube in the bank B of that group. Thenumber of heating tubes 55 is chosen as an odd number and consequentlythe outer tube 55" terminates near floor 18 of annular furnace section40 through which it is connected by an outlet pipe 57 which is in turnconnected to common outlet pipe 44 which is Secured to columns 13 andencircles heater 10.

Desirably, adjacent pairs of heating tubes 55 which are connected attheir upper ends are linked together by ,links 58 havingsimilar-Construction to links 52. Additionally, it is desirable to linkevery other pair of heating tubes 55 which are connected at their upperends by is afixed in roof 29. Thus swinging movement as well as verticalexpanson of the -upper ends of heating tubes 55 is permitted withinlimited amounts.

Each bank B of heating tubes 55 is further arranged such that thevertical plane passing through the axis of the heater 10 nwhich the bankB approximately lies passes medially between a pair of adjacent burners35 and therefore also medially between a pair of adjacent burners 41.Accordingly, it will be evdent that when burners 35 and 41 are Operatingto fire annular furnace section 40, each of the heating tubes 55 inbanks B are fired equally from both sides and receive substantiallyuniform heating. Additionally the tubes 55 in banks C are also radiantlyheated by direct firing but on one side only and accordingly receive asin the conventional Construction of furnaces of this type an uneven heatflux pattern. Since the fluid being heated in each pass through heater10' first. includesconvection heated tubes 47, then single firedradiantheated tubes 55 in banks C and finally double fired radiantheated tubes 55 in banks B, the fact that bank C-heating tubes 55receive an uneven heat flux pattern is of not a serious consequence asthe material being heated at this point does not reach temperatures atwhich uneven heating could cause such problems as coking and 'the like.It`wll he further observed that at the temperature endof each passthrough the furnace substantially uniform heat flux patterns areprovided, since the high temperature end of each pass includes theradial banks B which are double fired, and hence coking and otherproblems attributed -to neven heat flux are minimized. Banks C can beomitted, but are desirable since they are set at angles to theirrespective tube banks B and provide a more stable tube structure.

Furnace is generally operated in a conventional manner. Thus suitablefuel is supplied to burners 35, and burners 41 if additional heat in theupper section is desired, causing the burners 35 to fire upwardly intoannular section 40 and burners 41, if these are also employed, to fireacross annular section 40 up into upper section 42. The material to beheated, which can be gas oil, residual oils, hydrogen gas, or almost anyother fluid material, is introduced through header 43 and as indicatedabove flows serially through the eight passes in parallel, first passingthrough the convection heating tubes 47 in flue gas well 48, thenthrough the single fired radiant heated tubes 55 in the banks C, andlastly through the double fired radiant heated tubes in the banks B,thence into the common outlet pipe 44 from which they are passed to asubsequent refinery operation. Flue gases of course pass upwardlythrough annular furnace section 40 inwardly toward the center of upperfurnace section 42 and then downwardly through flue gas well 48 fromwhich they are withdrawn to stack 27 by blower 25 :through duet 22, box23 and waste heat boiler 24.

As suggested above tubes 55, because of their location in radiantlyheated furnace sections 40 and 42 are heated during operation of heater10 to substatially higher temperatures than tubes 47 located inconvection heated flue gas well 48. Tubes 55, consequently, expandupwardly as they are heated to Operating conditions proportonatelygreater than tubes 47. The ratio of lengths of inlet tubes 5'5' and ofoutlet tubes 47" is, accordingly, chosen to approximate the ratio of thetemperature differences to which the respective tubes are subjected inwarm up to typical Operating condition, such that their respective totalexpansions will be approximately equal. Since actual service conditionscan only be estimated in advance and since a given heater 10 maydesirably be employed in different service from time to time somedifference in total expansion of tubes 55' and 47" is to be anticipated.This difierence is tolerated in our heater construction by supportingall heating tubes in fixed position at their lower ends only and bysecuring the tubes at their upper ends in a manner permitting all tubesto move laterally at their upper ends within limited distances. Thuswarping of adjacent tubes, even of tubes 47" and 55', can be tolerated,tube fractures reduced, and tube life prolonged.

Referring more particularly to Figure 6 we have illus` trated amodification of a heater designed particularly to accommodatevaporization or partial vaporization of a liquid material in the doublefired heating tubes. In Figure 6 substantally the same constructcn isemployed and for the sake of Simplicity where the reference numeralscorrespond to those used with reference to the preceding figures theidentical Components are contemplated. Heater 100 shown in Figure 6diifers from heater 10 of the preceding figures only in the omission ofside 'wall burners 41 and in the construction and arrangement of heatingtubes. In heater 100 flue gas well 48 is provided with a cylindricalbattle 101 positioned vertically and coaxially within flue gas well 48extending entirely through the length of well 48 up into upper furnacesection 42. The purpose of bafiie 101 is to increase the speed of fluegases over convection heated tubes 102 which are located in flue gaswell 48 otherwise essentially in the same manner as tubes 47. Thus aninlet heater tube 102,' is connected to header 43 as described above andextends vertically ypward in flue gas well 48 and is connected at itsupper end to the next vertical tube 102 through a sequence in turn ofthirteen tubes 102 in each convection bank of serially interconnectedtubes 102 with the outlet convection heated tube 102" extended upwardlyinto upper furnace section 42. Each tube 102" is connected at its upperend with the inlet tube 103' of a radial bank D of a single row ofserially interconnected vertical heating tubes including an inner groupof five tubes 103 having a diameter essentially the same of thatconvection heated tubes 102 and an outer group of six tubes 104 ofsubstantially larger diameter. The outlet tube 104" of each bank D isconnected at its lower end to an outlet pipe 105 of even larger diameterwhich, like the outlet pipes 57 in the preceding figures, is connectedto common outlet pipe 106.

Thus the material to be heated passes through heater 100 including aninitial series of small diameter pipes 102 which are located in aconvection heated section in well 48, a second group of double firedsmall diameter tubes 103 in the radiant heated section of heater 100 andfinally a third group of double fired large diameter tubes 104 in theradiant heated section. Suitably heater 100 can be employed as a tubestill for preheating the feed to a vacuum flashing unit in which partialvaporization of the residual feed stock prior to flashing isaccommodated by the larger diameter heating tubes 104. Since a typicalvacuum flash unit desirably also includes steam stripping we provide inthe modification of Figure 6 a single pass of six larger diameter tubes107 which are positioned vertically and serially interconnected in fluegas well 48 adjacent to convection heated tubes 102. and adjacent tobaie 101. Tubes 107 suitably are employed to superheat stripping steamfor the vacuurn flashing unit.

` In the preceding discussion of the drawings little has been said ofthe phyical Construction of the heater tubes since these are fabricatedin the conventional manner and of conventional high temperature alloys,and are similar to those ordinarily use in refinery heater service. Thusthe convection heated tubes desirably are provided with longitudinallyextended fins of number and size such-that each of the convection heatedtubes lies substantially tangent to the next of its serialinterconnected band and tagent to those of adjacent banks. One feature,however, of the radiantly heated tubes should be ponted out. Wheredesign considerations indicate a larger number of tubes is a radiantlyheated bank B than can be ordinarily accommodated using standard sizereturn bends the arangement of Figure 7 is recommended which permits'adjacent double fired tubes in a given radial bank to be located asclose to each other as is required without staggering which introducesuneven heat flux distribution.

Thus in Figure 7 the upper end 55b of a pair of adjacent heater tubes 55located in a bank B is replaced by the Construction 110 shown in Figure7 in which a pair of adjacent tubes 55 are connected at their upper endsby construction 110. Construction 110 essentially con sists of asemi-circular standard return bend 111 which is disposed in a verticalplane set at an angle to the vertical radial plane of tubes 55. Returnbend 111 is connected at its ends by angularly disposed tube sections112 to the upper ends of the adjacent tubes 55. Connecting sections 112have lengths determined by the relative difference in diameter of bend110 and spacing of pipes 55, since sections 112 suitably turn intoreturn bend 111 and into heater tubes 55 with turns of the same radiusas standard return bend 111.

It will be evident from the preceding description that the oil refineryheater of our invention is adapted for a variety of refinery heatingproblems because of its accommodation to a wide range of outlettemperatures of the fluid being heated. It will also be evident that ourheater provides substantial economies in saving of eX pensive, hightemperature alloy heating tubes partly because of reduced tube stressdue to expansion, partly because double firing at the high temperatureend decregses 1 tube temperature for a given outlet temperature of thematerial being heated, and partly because double firing reduces thetotaltube area required to reach a given outlettemperature.

We claim:-

1. In an oil refinery heater including a vertical, cylindrical furnaceshell, a roof positioned across and closing the upper end of said'shell, a vertical, cylindrcal flue positioned centrally within saidfurnace shell, said flue terminating at the upper end thereof in saidshell below said roof and terminating at the lower end thereof below thelower end of said furnace shell, an annular floor positioned across thelower end ofsaid shell closing the lower end of said shell to said flue,whereby said heater includes an annular furnace space defined above saidfloor between said flue and said, shell, an upper furnace space definedwithin said shell between said roof and the open upper end of said flue,and a flue gas well' defined within said flue and having a lower portionbeneath the level of said floor, a plurality of burners mounted in saidannular floor spaced apart at arcuate intervals thereabout between saidfurnace shell and said flue for directing fuel and air upwardly intosaid annular furnace space and means for withdrawing flue gas from thelower portion of said flue gas well; the improvement which includes abank of a single row of serially interconnected heating tubes, each saidtube in said bank being disposed vertically extending through saidannular furnace space into said upper furnace space, said bank ofheating tubes being disposed approxinately in a radial plane withinsaid. cylindrical shell passing medially between an adjacent pair ofsaid burners, a heating tube disposed vertically within said flue gaswell interconnected at the upper end thereof in said upper furnace spaceto the upper end of the. innermost tube of said radial bank, saidheating tube within said flue gas well having a greater length than saidinnermost tube of said radial bank, and means for passing a fluid to beheated serially first through said heating tube disposed within saidflue gas well and then outwardly through said radial bank of heatingtubes.

2. In an oil refinery heater including a vertical, cylindrical furnaceshell, a roof positioned across and closing the upper end of said shell,a vertical, cylindrical flue positioned centrally within said furnaceshell, said flue terminating at the upper end thereof in said shellbelow said roof and terminating at the lower end thereof below the lowerend of said furnace shell, an annular floor positioned across the lowerend of said shell closing, the lower end of said shell to said flue,whereby said heater includes an annular furnace space defined above saidfloor between said flue and said shell, an upper furnace space defined:within said shell between said roof and the open upper end of said flue,and a flue gas well defined within said flue and having a lower portionbeneath the level of said floor, a plurality of burners mounted in saidannular floor spaced apart at arcuate intervals thereabout between saidfurnace shell and said flue for directing fuel and: air upwardly intosaid annular furnace space, and means for withdrawing flue, gas fromthe, lower portion of said flue gas well; the improvement which includesa plurality of banks of, single rows of serially interconnected heatingtubes, each said tube in said banks being disposed vertically extendingthrough said annular furnace, space into said upper furnace space, eachsaid bank of heating tubes being disposed approxmately in a differentradial plane in said cylndrical, shell, each said plane passing mediallybetween different adjacent pair of said burners, a plurality of heatingtubes disposedvertically within said, flue gas; Well, each said tube insaid well being nterconnected. at the upper end thereof in said upperfurnace space to the upper end of the innermost, tube of a different oneof said radial banks, said heating tubes within' said flue gas wellhaving a greater length than saidinnermost tubes of said radial banks to8 whichthey arerespectively connected, and means f! passing a fluid tobe heated serially first in Parallel throughsaid heating tubes disposedwithin said flue gas well and then in parallel outwardly through saidradial banks of heating tubes( 3. The improvement according to claim 2which further includes a plurality of banks of serially interconnectedvertical heating tubes disposed in said furnace spaces adjacent to theouter wall of said flue, each said last named bank being positionedinside of a different one oflsaid plurality of burners between anadjacent pair of said radial banks of heating tubes and beinginterconnected between the innermost tube in one of said adjacent radialbanks and the tube associated therewith in' said flue gas well.

4. The improvement according to claim 2 which further includes aplurality of burners mounted in said furnace shell, each positioned todirect fuel and air into the space between different adjacent pair ofsaid radial banks of said heating tubes above one of said burners insaidfloor.

5. In an oil refinery heater including a vertical, cylindrical furnaceshell, a roof positioned across and closing the upper end of said shell,a vertical, cylindrical flue positioned centrally within said furnaceshell, said flue terminating at the upper end thereof in said shellbelow said roof and terminating at the lower end thereof below the lowerend of said furnace shell, an annular floor positioned across the lowerend of said shell closing the lower end of said shell to said flue,whereby said heater includes an annular furnace space defined above saidfloor between said flue and said shell, an upper furnace space definedwithin said shell between said roof and the open upper end of said flue,and a flue gas well defined within said flue and having a lower portionbeneath the level of said floor, a plurality of burners mounted in saidannular floor and spaced apart at arcuate intervals thereabout betweensaid furnace shell and said flue for directing fuel and air upwardlyinto said annular' furnace space, and means for withdrawing flue gasfrom the lower portion of said flue gas well; the improvement which.includes a plurality of radial banks of single rows of seriallyinterconnected heating tubes, each said tube in said banks beingdisposed vertically and extending through said annular furnace spaceinto said upper furnace spaces, each said bank of heating tubes beingdisposed approxirnately in a difierent radial plane in said cylindricalshell, each said plane passing medially between different adjacent pairof said burners, a plurality of heating tubes disposed vertically withinsaid flue gas Well, each said tube in said well being intercon.- nected'at the upper end thereof in said furnace space to the, upper end of theinnermost tube of a different one of said radial banks, said heatingtubes within said flue gas well having a greater length than saidinnermost tubes of said radial banks to which they are respectivelyconnected, said tubes in said radial banks being firmly supported insaid floor at the lower ends thereof and spaced beneath said roof at theupper ends of said tubes to permit limited upward expansion thereof,said heating tubes in said flue gas well being supported at lower endsthereof in said lower portion of said well and spaced at upper ends ofsaid tubes beneath said roof to pernit limited expansion thereof, andmeans for passing a fluid to be heated serially first in paraJlelthrough said heating tubes disposed within said flue gas well and thenin parallel outwardly through said radial banks of heating tubes;

References Citerl in the file of this patent UNITED STATES PATENTS2,211,903 McCarthy Aug. 20, 1940 2,745,388 Becker May' 15, 19562,751,893 Permann June 26, 1956

